Typical audio device specifications include distortion specifications which are published as follows:

THD less than 0.02%
THD less than 0.02% at 1 KHz
THD less than 0.02%, 20 Hz to 20 KHz, 0 dBu, 300R load

The first specification is meaningless. It provides no information about the measurement conditions. The second is a little better acknowledging THD was measured at a single frequency with no level or load specified. It is not very useful unless you listen primarily to 1 KHz sinusoids. The third specification provides the frequency range over which the measurements were acquired, the voltage level and load used. This is a much more complete and honest specification. Yet greater insight may be derived from viewing the graph which produced that last specification.

THD adds the dB levels of 2nd and higher harmonics of the fundamental input sinusoid found in the FFT, and compares the result to the fundamental in the FFT.

THD+N adds dB levels of everything in the FFT except the fundamental, and compares to the fundamental, which typically results in a larger number due to inclusion of noise.

Here are some useful equations.

To obtain the graph a sinusoid input to the device is swept from 20 Hz to 20 KHz, measuring and plotting the THD and THD+N results from the device output.

Sample at 300 Hz :

Sample at 1000 Hz:

Sample at 3000 Hz.
The graphs for this device indicates a frequency neutral THD and THD+N. Residual noise observed is low as depicted in the spectrum with the primary residual noise being power supply mains related. However, given -90 dBu translates to 10 dB SPL for an HD800 this should remain inaudible.

A graph for the same device using a SMPS power supply:
The graph for this device indicates a frequency dependent THD with distortion rising as frequency falls. THD+N is frequency neutral but overall higher than the previous example. Also note the number of spikes in the spectrum. Residual noise (blackground) observed is noisier compared to the previous device measurement. (Please watch the y axis notation – this graph has different scales than the previous examples.)